Molecular genetic analysis of colon cancers has established that the Wnt signaling pathway is involved in early tumor development, particularly in colon cancer. The transcription factors that commute the signal into changes in target gene expression are members of the Lymphoid Enhancer Factor/T Cell Factor (LEF/TCF) family. The genes for two members of this family, TCF1 and LEF1, produce truncated dominant negative forms that limit Wnt signals and may function as growth suppressors. We have found that the LEF1 gene is ectopically induced in colon cancer, but only the full-length activating form is expressed - the dominant negative form is not. The patterns of LEF1 and TCF1 expression during colon cancer progression are not known. TCF1 is normally expressed in human colon, but the relative amounts of full-length and dominant negative forms in normal colon and cancer are also not known. Based on results from our preliminary studies, we propose that full-length isoforms of LEF/TCFs predominate in colon cancer. Furthermore, we propose that the Wnt pathway itself plays a role in promoting expression of full-length isoforms. To test these hypotheses we will determine the expression of LEF1 and TCF1 isoforms in normal, polyp and colon carcinoma (Specific Aim 1). To probe the mechanism of aberrant LEF1 expression, the genetic integrity of the LEF1 gene and its sensitivity to epigenetic changes in chromatin structure will be assessed in colon cancer cell lines (Specific Aim 2). TCFl/a-catenin and TCF4/ a-catenin complexes can activate the LEF1 promoter in transient transfection assays, other LEF/TCFs cannot. We propose that LEF1 is a target of specific TCFl/4/ a-catenin complexes in cancer. Experiments are proposed to explore this regulation and test for its relevance to the endogenous LEF1 gene in colon cancer cells (Specific Aim 3). Based on the selective effect of TCFI/4 on the LEF1 promoter, we hypothesize that each LEF/TCF family member carries unique target gene specificities and that their expression in cancer imparts distinct effects. To test this hypothesis, two strategies will be used to disrupt LEF/TCF function in colon cancer cells. Effects will be assessed by global gene expression profiles and cell growth and viability assays (Specific Aim 4).